The Lay Medical Man: 5/29/16

Friday, June 3, 2016

LEARN THE CONCEPT OF CRITICAL & PSEUDOCRITICAL TEMPERATURE WITH THE EXAMPLE OF ENTONOX

▪️Entonox is Nitrous oxide mixed 50:50 with oxygen

▪️It provides analgesia with maintenance of consciousness.

▪️Usually administered via a demand valve for self administration.

▪️Takes 30 seconds to act and continues for approx. 60 sec after inhalation has stopped

▪️ For optimum effect inhalation should start when the contraction tightens. This will co-ordinate the maximal effect with the central painful part of the contraction.

▪️20% N20 is equivalent to 15 mg of subcutaneous morphine.

▪️ The optimal analgesic concentration was found to be 70% but some mothers lost consciousness at this concentration

▪️ 50% N20 in oxygen is safer and this has become standard now

▪️ Entonox is the BOC trade name for this gas mixture.

▪️Poynting effect

The Poynting effect involves the dissolution of gaseous O2 when bubbled through liquid N2O, with vaporisation of the liquid to form a gaseous O2/N2O mixture.

▪️Critical & Pseudocritical temperature

🔹The critical temperature of a gas is the maximum temperature at which compression can cause liquefaction. Or it is the temperature above which a substance cannot be liquefied however much pressure is applied. Mixing gases may change their critical temperature.

🔹The pseudocritical temperature applies to a mixture of gases, such as Entonox, and is the temperature at which gas mixtures separate into their component parts.

▪️The Poynting effect produces a 50:50 mixture which reduces the crtical temperature of N20 so Entonox has a pseudocritical temperature of -6 degree.

▪️Entonox

Highest -5.5°C @117 bar

Cylinder -7°C @137 bar

Pipeline -30°C @4 bar

▪️In cylinders it is supplied at a pressure of 137 bar and must be stored above its pseudocritical temperature of -6°C.

▪️ Below this temperature the N2O liquefies in a process called lamination. If this occurs a high concentration of O2 will be delivered first with little analgesic effect, but as the cylinder empties the mixture will become progressively more potent and hypoxic as it approaches 100% N2O.

▪️If a cylinder has been exposed to cold below -6 degree C it should be warmed for 5 minutes in a 37 degree C water bath or for 2 hours in a room at 15 degree C. It should then be inverted three times before use.

▪️ When delivered via a pipeline at 4.1 bar the pseudocritical temperature is less than -30°C.

▪️Altitude per se has no effect on Entonox.

Reference: www.frca.uk

VEIN OF GALEN MALFORMATION: ANESTHESIA CONCERNS

▶️Vein Of Galen Malformation (VOGM) is a dilatation of the median vein of the procencephalon (which is a precursor of the Vein of Galen), caused by an arterio venous (A-V) shunt from the choroidal arteries of the anterior and posterior circulation. It can cause A-V shunting and related systemic effects and hydrocephalus.

▶️Incidence is less than 1% of cerebral vascular malformations.

▶️PROBLEMS:

✔️Most of the issues are due to the effect of A-V shunting

✔️Congestive Cardiac Failure and Pulmonary Hypertension : Due to A-V shunting . The etiology of cardiac failure may begin in utero. High output cardiac failure is associated with VGAM because 60 to 80% of aortic blood flow is directed through the VGAMS low resistant shunt. Blood flow travels through the VGAM during diastole creating a steal phenomenon. The reduction of diastolic pressure can lead to myocardial ischemia secondary to reduced coronary blood flow.

✔️Ventriculomegaly : A-V shunting --> increased dural sinus pressure --> resistance to CSF entry to the sinus --> enlargement of ventricles

✔️Seizures

▶️INDICATIONS FOR DEFINITIVE MANAGEMENT: CCF, Hydrocephalus , Neurological symptoms

▶️TREATMENT

✔️Main goal of the treatment is to control the A-V shunt

✔️Endovascular embolization is the preferred treatment; other option is surgical clipping.

✔️Embolization of the feeding arteries and draining veins can result in reduction of blood flow through the VOGM which is the key to improved cardiac function and brain injury prevention.

✔️Embolization is less invasive and has a higher survival rate than open neurosurgical procedures. It provides better hemodynamic stability with minimal pain. Heart failure is an indication for urgent embolization.

▶️ANESTHESIA CONCERNS

✔️Improving the cardiac function is the key to avoid multiple organ failure. The mortality rate for all neonates in heart failure undergoing transcatheter embolization can be up to 50%, mortality is much higher when pulmonary hypertension is present.

✔️Cardiac failure is difficult to treat and most beta agonists along with diuretics and Milrinone have been used with varying success. Although total systemic vascular resistance is reduced through the VOGM, the child in cardiac failure has an increased extracranial vascular resistance. So interventions that reduces extracranial systemic resistance is likely to improve systemic perfusion. Nitric oxide has been used to treat pulmonary hypertension in these patients.

✔️The goal is to achieve partial embolization with the endpoint of improving heart failure. So, the embolization may be staged. Each session is limited by the volume of contrast media delivered and the patient’s tolerance of the procedure. The ultimate goal of these sessions is to completely occlude the VOGM while avoiding neurological and cardiac injury

✔️Intracerebral hemorrhage due to venous hypertension is a potentially fatal complication of endovascular management. There is a thought that this complication can be avoided by staging the embolization procedure. Perforation of the venous sac can usually be managed by reversal of anticoagulation and continuation of coil embolization. Ischemic neurological deficits, Pulmonary embolization with embolic agents ( due to the high flow across the intracranial shunt that drains immediately into the central venous system) are other concerns.

Reference:

Anesthetic Implications of Neonatal Vein of Galen Aneurysmal Malformation (VGAM) Ira S Landsman, Than Nguyen